There exists in medicine an important and continuing need to quickly and accurately image both blood and tissue in order to facilitate proper diagnosis and treatment of various medical conditions. A prior art technique of providing an image of a lumen employs X-ray fluoroscopy. In X-ray fluoroscopy, a contrast agent is sent through an artery of interest. The contrast agent is visible under X-ray radiation which enables an X-ray system to provide an image of the arterial obstruction. However, such X-ray imaging techniques have disadvantages. An X-ray image of an arterial obstruction is a profile of the contrast agent as it flows through the artery. Therefore, the images provided are generally of the contrast agent in a single plane of view, not of the tissue. Moreover, the true lumen diameter is generally not provided by these images. In addition, the characteristics of the plaque within the artery are not provided, which is important in determining the success of a possible angioplasty procedure. Another disadvantage is the potential harmful effects of the radiation to the patient and attending medical personnel. Furthermore, there is potential for additional harmful effects due to the contrast agent.
In prior art ultrasound imaging systems, a transducer is utilized that emits an ultrasonic imaging beam. The transducer may be fastened, for example, about the outside circumference of a catheter and the ultrasonic imaging beam emanates perpendicular to the catheter. Because of this, the transducer only provides an image of areas on the sides of the catheter, as shown in FIG. 9. Such prior art catheter side scanning systems generally do not provide an image of the central region of the lumen in front of the catheter tip. This is a disadvantage since it is the central region of the lumen that should be imaged in order to characterize a severe stenosis. These systems can produce volumetric images by moving the catheters along the vessel, storing the 2D images, and then constructing a volumetric image, but not in real-time and only in a cylindrical volume around the path of the catheter.
In addition, such prior art side scanning systems proceed blindly forward through the blood vessel as the catheter is moved. This forward, unchecked motion may inadvertently cause the catheter to contact the blood vessel wall and shear off material attached to the blood vessel wall so that it is pushed into the bloodstream. Furthermore, such prior art side scanning systems generally do not provide for the guidance of therapeutic procedures such as laser ablation or mechanical atherectomy. Moreover, prior art catheter side scanning systems provide a plane image on the side of the catheter, which prevents a substantial portion of many pathologies from being characterized.
Another prior art technique utilizes a catheter that is volume imaging and front looking but does not employ ultrasonic imaging techniques. U.S. Pat. No. 4,998,916 to Hammerslag, et al discloses a steerable catheter device for coronary angioplasty applications. The device can negotiate the tortuous character of a vascular system. Fiber optic bundles are located at the tip of the device that illuminate an area in front of the device. In order to visualize a volume, a transparent inert liquid, such as a saline solution, must be discharged into the vascular system. The transparent liquid is discharged in front of the device and displaces blood from the front of the device. This enables a user to view through the liquid and observe the volume in front of the device.
However, systems employing fiber optics have disadvantages. One disadvantage is that essentially only the surfaces of the pathologies can be seen. In addition, the liquid utilized must be replaced frequently since it will dissipate and is absorbed into the vascular system. Therefore, the amount of time that this prior art device can be used is dependent upon the ability of the patient's vascular system to absorb the liquid. Moreover, this technique is not particularly reliable and is time consuming to use and therefore relatively expensive.
Still another technique is to use a transducer nutatably mounted on the tip of a probe for luminal scanning of an oncoming area or volume as the probe moves to a selected position in a lumen or body cavity. Such a front mounted transducer provides a scanning system that is forward looking and that uses ultrasound for intraluminal imaging. U.S. Pat. No. 5,373,845, entitled "Apparatus And Method For Forward Looking Volume Imaging", issued Dec. 20, 1994, to Gardineer et al., and incorporated herein by reference, discloses such a scanning system. In this patent, the beam is produced by a nutating single element transducer operating in a pulse mode and centered at a single frequency and obtains imaging data one ray at a time. Such an imaging method, however, results in an undesirably slow process. For example, use of the single element transducer for a 10 centimeter depth of field and 2,000 pixels per image would require 0.33 second per volume, which is not useful for real time visualization. Prior art does suggest using a wide angular spectrum of illumination to image. While the prior art, which includes the area of acoustical holography, suggests using grating-like structures to form multiple illuminating beams, the prior art fails to provide a useful and rapid image formation mechanism by which to generate full 3-D volume images. Thus, it is greatly desirable to increase the speed at which the image is formed in order to provide a more useful ultrasound imaging system.
Three-dimensional realtime ultrasound imaging systems have been devised. In particular, real-time ultrasound volumetric imaging has been performed by using multiple beams at a time. Von Ramm et al. U.S. Pat. No. 4,694,434, teaches the use of a 2-dimensional phased array with the transmission of a single beam and multiple "receive beams" for multiple transmit beams and one or more receive beams) to acquire the data required for 3D reconstruction in real time. This apparatus requires a 2-dimensional phased array structure, as well as phase-controllable receive channels on each element of the array. Such a system is expensive, and the number of cables required for the 2D phased array would preclude use in an internal, catheter-based imaging system.
U.S. Pat. No. 5,720,708 entitled HIGH FRAME RATE IMAGING WITH LIMITED DIFFRACTION BEAMS discloses the use of multiple simultaneous beams from a single pulse and decodes the echoes from the multiple beam so as to obtain multiple imaging points. In similar fashion to U.S. Pat. No. 4,694,434, this patent also requires a two-dimensional array where each individual element in the two-dimensional array must be connected to individual channels requiring a plurality of cables and complex interconnections. The decoding of the signal containing the multiple beam reflections also requires complicated mathematical processing involving two-dimensional Fourier transforming. Moreover, the field of view is limited to approximately the size of the array, thereby limiting the use for invasive internal body imaging. Accordingly, an apparatus and process for generating full 3-D volume images in a rapid and efficient manner which minimizes the cable interconnections, as well as simplifying the signal processing and filtering and which allows examination of a volume or object which is much larger than the imaging system itself, is greatly desired.